Pentaquark Θ+ search at HERMES

Abstract

The earlier search at HERMES for narrow baryon states excited in quasireal photoproduction, decaying through the channel pK0S→pπ+π−, has been extended with improved decay-particle reconstruction, more advanced particle identification, and increased event samples. The structure observed earlier at an invariant mass of 1528 MeV shifts to 1522 MeV and the statistical significance drops to about 2σ for data taken with a deuterium target. The number of events above background is 68+98−31(stat)±13(sys). No such structure is observed in the hydrogen data set.